During human development the structural components of the contractile apparatus of muscle change. Accordingly, distinct proteins encoded by members of multigene families are present in muscle at embryonic, neonatal and adult stages of development. Changes in muscle protein isoforms occur in the course of cell differentiation, ontogeny, and in response physiological stimuli and these changes have a marked impact on the contractile properties of the muscle. Complex regulatory mechanisms must exist that determine that the correct protein is expressed in the correct place at the correct time. The plasticity of gene expression during muscle development is especially well demonstrated by the number and diversity of myosin heavy chain proteins (myosins). A major advance in the study of the expression of human myosins in culture has resulted from the use of the fluorescence-activated cell sorter. Now, pure populations of primary human muscle cells can be isolated in quantity from muscle tissues at all stages of human development and differentiated in culture. In addition, the regulation of human myosin gene expression can be monitored in heterokaryons, cell fusion products in which dormant human muscle genes are activated. A developmental progression in the expression of several myosin genes has recently been detected in cultured human muscle cells with monoclonal antibodies, highly sensitive and specific reagents that recognize distinct human myosins. These transitions in muscle gene expression are now amenable to a biochemical and molecular analysis. The specific aim of this proposal is to increase our understanding of the developmental regulation of human muscle proteins. Toward this end, the sequence of human myosins expressed in vivo will be characterized and induced in vitro. The developmental regulation of myosin gene expression by putative stage-specific trans-acting factors will be explored in heterokaryons. The cDNAs specific to developmental and fiber-type specific myosins whose expression has been analyzed with monoclonal antibodies will be isolated and characterized. This approach should ultimately lead to the dissection of the regulation, structure and function of members of the myosin heavy chain gene family at a molecular level in the course of human development.